Field of the Invention
FIG. 1 is a diagram showing a configuration of a conventional device. The conventional device is arranged so that a first DC/DC converter 10 (DC/DC converter 1) and a second DC/DC converter 20 (DC/DC converter 2) are connected in parallel with a load via diodes for parallel operation so that power can be supplied to the load.
When a switching element TR1 is ON, an input voltage E is switched by the switching transistor TR1. C1 is a condenser for noise elimination connected across the input voltage E. As shown, an FET is used as the switching element TR1. A primary winding of a power transformer T is connected as a load of the FET. The switching element TR1 is driven by a control part (CONT) 1. For example, PWM (pulse width modulation) is used as a driving method. That is, by controlling the ON time in a clock cycle, the switching element TR1 is controlled so that an output voltage E01 is kept constant.
A high-frequency alternating current introduced on a secondary side of the power transformer T when the switching element TR1 is ON is converted into a direct current voltage by means of a full-wave rectifying circuit made up of diodes D1 and D2. The direct current voltage is applied to a smoothing condenser C2 via a smoothing coil L1, and is regulated so as to be a flat direct current voltage. R1, R2 and R3 are resistors connected between an output line and a common line, and have th function of dividing the output voltage. R4 is a resistor connected between the output line and the common line.
The output voltage supplies power to a load 2 via a diode D3 for parallel operation (parallel connection). The output voltage obtained at that time is divided by the voltage dividing circuit made up of the resistors R1.about.R3, and a divided voltage is applied to the control part 1 as the output voltage. The control part 1 compares the output voltage with a reference value and controls the ON time of the switching element TR1 so that both the values coincide with each other (PWM control).
When the switching element TR1 is OFF, energy stored in the smoothing coil L1 passes through a loop of smoothing condenser C2.fwdarw.parallel operation diode D3.fwdarw.load 2 .fwdarw.and diode D2, and current is supplied to the load 2.
The foregoing is directed to the operation of the DC/DC converter 1, and an operation of the DC/DC converter 2 is the same as the above. Power (current I0) is supplied to the load 2 by the two DC/DC converters.
In the above-mentioned circuit, it is necessary to add block diodes (diodes for parallel operation) in order to prevent, when one of the DC/DC converters becomes abnormal at the time of parallel operation, the other DC/DC converter from being affected.
In the above-mentioned circuit, power is supplied to the load 2 via the parallel operation diodes D3. Thus, if the load current increases, the voltage drop developing across the diode D3 increases, and the output vs. voltage characteristic obtained at this time is as shown in FIG. 2. As the output current I0 increases, the output voltage E01 is decreased due to the voltage drop developing across the diode D3, and is dropped by .DELTA.V1 about the rated current. Thus, the output voltage at that time becomes as shown in FIG. 2, and a drop of the output voltage may prevent a communication device serving as the load from normally operating.
The present invention is made taking into consideration the above problems, and has an object of providing a parallel operating device of a DC/DC converter capable of suppressing a drop of the output voltage at the time of parallel operation.